Back pain is major problem in the adult population. The pain may have multiple causes, whereof some may require surgery. Lower back pain may be caused by displacement of the vertebrate bodies and the intermediate discs in the lumbar region of the spine and particular L4-L5 and L5-S1 are vulnerable. For patients with severe pain that doesn't respond to conservative treatment, fusion surgery may be an option. Spinal fusion surgery (fusing one vertebra to another) is often done to decrease motion at a painful motion segment to reduce associated pain at that segment. This abnormal and painful motion can be caused by painful discs (discogenic pain or degenerative disc disease), abnormal slippage and motion of the vertebra (spondylolisthesis or spondylolysis), or other degenerative spinal conditions, including but not limited to facet joint degeneration. In addition, a spine fusion may be indicated for any condition that causes excessive instability of the spine, such as certain fractures, infections, tumors, and spinal deformity (such as scoliosis).
Several treatment methods are known but further improvements are desired as the different methods all are associated with disadvantages.
During posterolateral spine fusion (PLF) surgery a graft is laid out in the posterolateral portion of the spine. Interbody surgeries may be performed either from the front or from the back and are thus described as Posterior lumbar interbody fusion (PLIF), Transforaminal lumbar interbody fusion (TLIF) and Anterior lumbar interbody fusion (ALIF). The different types of operation include removing the disc between two vertebrae and inserting bone into the space created between the two vertebral bodies. Posterior surgery leads to acceptable results and is claimed to further improve outcome by adding anterior column support as can be achieved by ALIF, TLIF or PLIF. The combined fusion procedures are generally defined as Circumferential fusion. These types of operations, where posterior stabilisation is needed, are unfortunately associated with a long recovery compared to exclusively anterior surgery.
In a further used technique the invertebra disc is replaced by an implant attached to the vertebra bodies above and below. Following surgery bone tissue grow around the implant and thereby fusion with the vertebra bodies is obtained.
The position of the vertebra bodies is decided during surgery by the fixation used or partly by the design of the implant used. Currently three types of Total Disc Replacement (TDR) implants have been used. Unconstrained designs appear to have some advantages as they are more likely to provide a physiologic mobile instantaneous axis of rotation (IAR), thus displaying a greater range of motion in vivo. Their lack of constraint may prevent excessive facet joint or capsuloligamentous loads in the extremes of flexion and extension. Furthermore, since the IAR is mobile, they may be less sensitive to small errors in implant placement. On the other hand, constrained devices appear to have an advantage in protection of the posterior elements from shear loading. Spinal shear loads of considerable magnitude occur during activities of daily living. A third group of implants are characterised as semi constrained implants including Prodisc, Maverick and Flexicore and are currently in use.
In general the position of the disc implants is determined during surgery as the fusion requires stabilization until bone growth has occurred which may often take several months (3-6 months). If the position is not correct the surgery may be inefficient or may even result in secondary effects caused by stress of the neighbouring discs. Subsequent surgeries are complicated by the previous surgery.